This invention generally relates to electronic systems, and more particularly it relates to audio power amplifiers.
A prior art audio power amplifier with PMOS output driver is shown in FIG. 1. The device of FIG. 1 includes amplifier 20, PMOS output driver 22, NMOS output driver 24, output coupling capacitor 26, and resistance 28 which represents a speaker. The backgate diode node 30 of PMOS output driver 22 is connected directly to the supply voltage Vdd. During normal operation, supply voltage Vdd is greater than output voltage Vo. Thus the backgate diode of transistor 22 is normally reverse biased. However, during power down, output voltage Vo can exceed supply voltage Vdd due to the charge stored on capacitor 26. The backgate diode of transistor 22 then becomes forward biased. This can cause the output coupling capacitor 26 to rapidly discharge, creating an audible pop sound at the speaker 28.
Generally, and in one form of the invention, a circuit includes: a transistor; and a switch coupled to a backgate of the transistor for switching between a power supply node and current-path node of the transistor to prevent a voltage on the backgate from dropping below the voltage on the current-path node.